It is well known that moisture has adverse effects on the performance of moisture sensitive devices like organic light emitting diodes and organic photovoltaics. Therefore, barrier stacks comprising organic and inorganic layers are used to encapsulate such sensitive devices and protect them from permeation of moisture.
EP2445029A1 discloses multilayered protective layers for organic opto-electronic devices for example in organic light emitting diodes (OLED). The cathode in an OLED is usually covered by a thick aluminum layer, which acts as a barrier against water. However, due to pinhole defects in such an aluminum layer water penetrates into the cathode layer and oxidizes the metal at the cathode-polymer interface, preventing electron injection from the cathode into the polymer during operation of the device. As a consequence, black spots appear in the bright field of electroluminescence. By applying an organic layer on top of the OLED, the number of black spots can be decreased but there is still a significant number of black spots appearing over time, which affects the use and hence the lifetime of a device. If this organic layer contains moisture absorbing particles, it helps to bind the moisture in the organic layer. A typical concentration of the particles as used in the organic layers of above patent application is about 5 wt %.
Because of the high concentrations of the water absorbing particles, light scattering at the particles of an organic layer causes generation of haze when illuminated with visible light. As long as such layers are, for example, on an OLED device but on the side, which does not emit light, light scattering is not a problem. On the other hand, if the layer is on the light emitting side of an OLED, then haze is very often unwanted and there is the requirement to reduce haze occurrence.
U.S. Pat. No. 8,044,584B2 describes an organic electroluminescent device and compositions with both metal oxide and metal salt particles, a binder and a dispersant for the preparation of moisture absorption layers. It teaches that the moisture absorption ability decreases if the concentration of metal oxide and/or metal salt is less than 2 wt % with regard to the amount of binder. The inventors propose to avoid haze by reducing the average particle diameter of metal oxide and metal salt particles to less than 100 nm. A dispersant is additionally used in the composition to avoid aggregation of the particles in the final moisture absorption layer. Because of the small particle size scattering of visible light does not occur. However, production of particles with an average diameter less than 100 nm is challenging and adds production costs.
With the continuous demand to improve the barrier layer performance without worsening the optical performance of devices using such layers, there is a need to develop improved compositions, which overcome the existing problems. There is also a continuous demand to reduce the manufacturing costs of the organic layer composition without sacrifying the electro-optical performance of a device for which it is employed.